The 12 and 24 month bars are rules on whether or not a J-1 scholar is eligible for a second visit to the U.S. in the J-1 Professor or Research Scholar category.
A two-year (24-month) bar on repeat participation in the J-1 Professor or Research Scholar categories was instituted by the U. S. Dept. of State (DOS) for exchange visitors upon completion of their J-1 programs. This means that all J-1 Professors or Research Scholars (not those in the Short-term Scholar category) will be barred from participation in the same category for two years from the end date of their programs. For example, a professor whose J-1 program is from September 1, 2008 - May 31, 2009, will not be able to return to the US as a J-1 Professor (or Research Scholar) until May 31, 2011. (Note: this is a different rule from the two-year home residency requirement.)
In addition to the 24-month bar on repeat participation, J-1 regulations prohibit participation of any exchange visitor coming for a new program in the category of Professor or Research Scholar, as indicated in #4 of the enclosed DS-2019 form, who has been physically present in the U.S. in ANY J status during any portion of the 12-month period immediately preceding the date of program commencement set forth on her/his new form DS-2019 with the following exceptions: 1) the new program is a transfer from another program, 2) presence in the U.S. in the previous program was for less than six months, and 3) presence in the U.S. in the previous program was under the category of short-term scholar.
The charts on the right shows how long you must wait before you are eligible to begin a new J-1 Research Scholar or Professor based on your previous J-1 status.